The field of the invention is magnetic resonance imaging (“MRI”) systems and methods. More particularly, the invention relates to systems and methods for tracking an interventional device that can be actuated to induce measurable susceptibility effects.
The placement of interventional devices, such as guidewires and stents, using MRI guidance is a promising and evolving field with great clinical potential. One particular challenge of this field, however, has been how to develop safe and reliable methods for tracking such devices as they are moved and manipulated within vessels or organs. The tips of guidewires can be easily visualized using conventional x-ray fluoroscopy by applying small, radio-opaque markers to the tips. In MRI, the analog to the radio-opaque marker is a marker made of a material with a sufficiently large magnetic susceptibility, relative to the surrounding tissues, such as a stainless steel tip on a nitinol wire. In MR images depicting a guidewire containing such markers, a local hypointense region is present in the tissues adjacent to the markers, thereby resulting in a loss of clinically relevant information. Exemplary MR visible interventional instruments of this kind are described, for example, in U.S. Pat. Nos. 5,728,079 and 6,430,429.
The interventional instrument described in U.S. Pat. No. 5,728,079 is a catheter provided with a hollow tubular holder, and in which the indicator element includes a concentric layer of a paramagnetic material. The concentric paramagnetic layer is provided in the form of a cylindrical sheath whose longitudinal axis is coincident with the longitudinal axis of the holder. The paramagnetic material influences the magnetic resonance image of a patient to be examined by means of an MRI system. The influence the device has on MR images makes it possible to determine the position of the interventional instrument within the body of the patient without the instrument being directly visible. However, the influence of the paramagnetic component in the device on the MR image adversely affects the diagnostic quality of the magnetic resonance image. As a result of the influence of the indicator element, MR images will exhibit degraded or lost anatomical details in the regions adjacent to the indicator element.
The interventional instrument described in U.S. Pat. No. 6,430,429 includes an indicator element for which the degree of influencing of the magnetic resonance image is adjustable, notably by rotation of the indicator element relative to the direction of the steady magnetic field of the MRI system. For example, the indicator element is a paramagnetic strip which may include several segments of different magnetic susceptibility. Only paramagnetic components are described in U.S. Pat. No. 6,430,429. Thus, the influence of this device on a magnetic resonance image will depend on the orientation of the device relative to the magnetic field of the MRI system. However, this adjustability of the device's influence is limited, and while the degree of the device's influence on MR images can be reduced, it cannot be eliminated and signal loss will still occur even with this reduced influence.
Despite being easy to locate in MR images and relatively inexpensive and safe, the aforementioned interventional devices produce a loss of signal in the vicinity of the indicator element that, in turn, obscures the desired region of interest: the tissue adjacent to the tip of the device. Thus, while the location of the tips of the aforementioned devices can be easily identified, the nature of the tissue that the devices are being moved through is obscured by the same effect that allows the visualization of the devices.
It would therefore be desirable to provide an interventional device that can be accurately located in an MR image while also allowing visualization of the tissue adjacent to the device by mitigating signal losses.